How to work out subscription revenue over 12months

Hello again

I am putting together a cash flow forecast on excel. Now there is a subscription element to this. I need a formula that will calculate the revenue from a customer over the length of the subscription (12 months)

so if in January I sign up 10 people who pay £5 a month for 12 months, and in February I sign up 20 who pay £5 for the next 12 months ect, I need a formula that will add on the new subscriptions and work out the expected cash inflow. Because the cash flow is for a 3 year period I also need the formula to take off the subscriptions after they run out in 12 months. I thought I could do it by adding that months new subscribers revenue to the previous months and taking away the revenue from the same month the previous year (To take away those customers who's subscriptions have ended) but I started to get negative numbers which really shouldn't happen

I've attached a sample spreadsheet of how it will be displayed.

Thank you in advance